1. Field of the Invention
The present invention relates to a toner for electrostatic image development, to be used for image formation in a copier, printer or other electrophotographic system.
2. Description of the Related Art
In image forming methods using electrophotographic systems in general, an electrostatic latent image is formed on a photosensitive member, the electrostatic latent image is then developed with a toner, and the resulting toner image is transferred either directly or indirectly as necessary to a transfer paper or other recording medium and fixed to obtain a visible image.
In recent years there has been increasing demand for higher printing speeds and greater resolution and image quality in printers and the like, and attempts have been made in the field to achieve greater image quality by reducing the size of the toner particles. This is particularly notable in the case of color toner, and toner particle sizes are being made smaller and smaller due to the appearance of toners prepared not only by dry methods, but also by wet methods such as the suspension polymerization method, agglomerated particle method and dissolution suspension method.
From an environmental perspective, on the other hand, printers and the like are subject to demands for energy savings. Reducing the fixing energy is especially important, and as a countermeasure for this, methods of reducing the toner laid-on level on the recording medium are being actively studied. Increasing the tinting strength of the toner is key to achieving this.
The tinting strength of the toner can be increased by increasing the added amount of the coloring agent or improving the dispersibility of the coloring agent in the toner, but coloring agents are normally expensive, so the problem with the first method is that it may increase the raw material cost of the toner. If a large amount of coloring agent is added, moreover, the intrinsic charging performance and polarity of the coloring agent are more likely to affect the toner, adversely affecting the charging performance of the toner, and detracting from the granulating properties in some cases in the case of toners formed by wet methods. There has therefore been much research into improving the dispersibility of the coloring agent in the toner, and for example a method has been proposed for surface treating the pigment (Japanese Patent Application laid-open No. H11-119461).
However, there is room for improvement in the dispersibility of the pigment in the toner. In the case of toner particles that are polymerized in an aqueous medium, moreover, the pigment can become overconcentrated on the surface of the toner particles due to the presence of polar groups on the pigment surface, detracting from the charging performance and stress resistance.
To improve the tinting strength of the toner, it is necessary first and foremost to pulverize the pigment as finely as possible, and disperse it uniformly in a binder resin. To this end, in the case of toner particles obtained by suspension polymerization for example, the pigment must be uniformly and finely dispersed in a polymerizable monomer before being polymerized.
However, in the case of toner particles obtained by the suspension polymerization method or dissolution suspension method, it is difficult to achieve uniform and fine dispersion of the pigment because there is no step of uniformly mixing a toner material with strong shearing force using a highly viscous medium, as when melt-mixing toner obtained by a pulverization method.
Therefore, a method using various kinds of media dispersers has been proposed as a method of disposing a pigment in a polymerizable monomer (Japanese Patent laid-open No. 2005-77729).
However, even if the pigment is uniformly dispersed in a polymerizable monomer using various kinds of dispersers before the granulating step, dispersion of the pigment particles in the liquid is not stable, and it is often the case that the pigment re-aggregates during the granulating step or reaction step, or becomes overconcentrated at the boundary between the water and the toner particle oil droplets. On the other hand, if the pigment is insufficiently dispersed in the polymerizable monomer composition, it is difficult to form uniform liquid drops of the polymerizable monomer composition in the aqueous medium, and in some cases the particle distribution of the toner particles may become too broad, the image density of the resulting toner may be reduced, and the resolution may be seriously affected.
Another problem has been that the charging performance and stress resistance of the toner declines when the pigment is overconcentrated on the surface of the toner particles.
As a method of addressing these problems, the use of various pigment dispersants has been studied for improving the dispersibility of the pigment in the toner (Japanese Patent laid-open No. 2010-152208). Although the dispersibility of the pigment can be temporarily increased by this means, this is not sufficient to stabilize dispersion in a polymerizable monomer or other liquid. In particular, when toner particles are manufactured by the suspension polymerization method or dissolution suspension method, stress resistance and charging performance are often achieved by forming a shell layer with a polar resin on the surface of the particles. In this case, the pigment dispersant may act on the polar resin rather than on the pigment in the dispersion step, granulation step or reaction step, so that the desired effect on dispersion of the pigment is not obtained. This may also result in insufficient shell layer formation on the toner, making it difficult to closely control the charging performance of the toner. The stress resistance of the toner may also be adversely affected, so that a stable high image quality cannot be maintained during long-term use.
In all of these methods, it has been difficult to disperse the pigment in the toner and obtain a toner with improved tinting strength without adversely affecting the manufacturing stability, charging performance and stress resistance of the toner when manufacturing a toner by the suspension polymerization method or dissolution suspension method.